Antibodies of high specificity toward human lymphoblastoid interferon have been produced in sheep and rabbits. Antibodies to interferon have also been produced by in vitro cultures of lymphocyte hybridomas. Experiments are now in progress to clone the hybrid cell lines and produce monoclonal IgG or IgM for use in radioimmunoassay and receptor studies. Using 3H methylated and 125I Bolton Hunter labeled interferon as labeled antigen, standard curves for quantitative immunoprecipitation assays have been made. With the present specific radioactivity of labeled interferon the radioimmunoassay can be sensitive to within 10-100 Units of interferon.